A typical investment casting process involves the production of engineering metal castings using an expendable pattern. The pattern is a complex blend of resin, filler and wax which is injected into a metal die under pressure. Several such patterns, once solidified are assembled into a cluster and mounted onto a wax runner system. The wax assembly is dipped into a refractory slurry consisting of a liquid binder and a refractory powder. After draining, grains of refractory stucco are deposited onto the damp surface to produce the primary refractory coating (the covering of the assembly with refractory material is known as “investing”, hence the name for the process). When the primary coat has set (usually by air drying until the binder gels) the assembly is repeatedly dipped into a slurry and then stuccoed until the required thickness of mould shell is built up. Each coat is thoroughly hardened between dippings, and so each mould can take from between 24 and 72 hours to prepare. The purpose of the stucco is to minimise drying stresses in the coatings by presenting a number of distributed stress concentration centres which reduce the magnitude of any local stresses. Each stucco surface also provides a rough surface for keying in the next coating. The particle size of the stucco is increased as more coats are added to maintain maximum mould permeability and to provide bulk to the mould.
In recent years, advanced ceramics (eg. silicon nitride) components have been developed which offer significant advantages over comparable metal components. Many processes by which such ceramic components can be made are known, and these include machining, injection moulding, slip casting, pressure casting and gelcasting. In gelcasting, a concentrated slurry of ceramic powder in a solution of organic monomer is poured into a mould and polymerised in situ to form a green body in the shape of the mould cavity. After demoulding, the green ceramic body is dried, machined if necessary, pyrolysed to remove binder and then sintered to full density. Aqueous based systems, such as the acrylamide system, have been developed in which water-soluble monomers are used, with water as the solvent.
It is an object of the present invention to provide an improved investment casting process which obviates or mitigates one or more problems associated with known investment casting processes and which preferably significantly reduces the time required for forming a shell mould.